Mosfets comprising source/drain recesses with slanted sidewall surfaces, and methods for fabricating the same

ABSTRACT

The present invention relates to improved metal-oxide-semiconductor field effect transistor (MOSFET) devices with stress-inducing structures located at the source and drain (S/D) regions. Specifically, each MOSFET comprises source and drain regions located in a semiconductor substrate. Such source and drain regions comprise recesses with one or more sidewall surfaces that are slanted in relation to an upper surface of the semiconductor substrate. A stress-inducing dielectric layer is located over the slanted sidewall surfaces of the recesses at the source and drain regions. Such MOSFETs can be readily formed by crystallographic etching of the semiconductor substrate to form the recesses with the slanted sidewall surfaces, followed by deposition of a stress-inducing dielectric layer thereover.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/427,491, filed Jun. 29, 2006.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices that can be usedin complementary metal-oxide-semiconductor (CMOS) circuits. Morespecifically, the present invention relates to an improvedmetal-oxide-semiconductor field effect transistor (MOSFET) comprisingsource and drain (S/D) recesses with slanted sidewall surfaces and astress-inducing dielectric layer that is located over the slantedsidewall surfaces of such recesses, as well as methods for forming suchS/D recesses by crystallographic etching.

BACKGROUND OF THE INVENTION

Mechanical stresses within a semiconductor device substrate have beenwidely used to modulate device performance. For example, in silicon,hole mobility is enhanced when the channel film is under compressivestress in electrical current direction and/or under tensile stress in adirection normal of the silicon film, while the electron mobility isenhanced when the silicon film is under tensile stress in electricalcurrent direction and/or under compressive stress in the directionnormal of the silicon film. Therefore, compressive and/or tensilestresses can be advantageously created in the channel regions of ap-channel field effect transistor (p-FET) and/or an n-channel fieldeffect transistor (n-FET) in order to enhance the performance of suchdevices.

One possible approach for creating a desirable stressed silicon channelregion is to cover the FET devices with compressively and/or tensilelystressed dielectric films, such as silicon nitride films. For example,U.S. Patent Application Publication No. 2003/0040158 published on Feb.27, 2003 for “SEMICONDUCTOR DEVICE AND METHOD OF FABRICATING THE SAME”describes a semiconductor device that contains a first tensilelystressed nitride layer overlaying the channel region of an n-MOSFET, anda second compressively stressed nitride layer overlaying the channelregion of a p-MOSFET, for respective application of tensile andcompressive stresses to the n-MOSFET and p-MOSFET. However, suchoverlaying stressed nitride layers can only create a limited amount ofstress in the channel regions of the MOSFET devices.

Recessed source and drain regions have been used in conjunction with thestressed nitride layers to improve the stress profile in the MOSFETchannel region. Specifically, the source and drain regions of the MOSFETdevices are etched back to form recesses with vertical sidewalls and asubstantially flat bottom surface. A stressed nitride layer, which isformed not only on top of the MOSFET channel region but also in thesource and drain recesses and over the vertical sidewalls of the MOSFETchannel region, is significantly more effective in creating stress inthe channel regions, in comparison with a similar nitride layer that isformed only on top of the MOSFET channel regions. However, such recessedsource and drain regions with vertical sidewalls undercut the source anddrain extension regions in the MOSFET, which leads to increased shortchannel effects, increased junction leakage, and deteriorated deviceperformance.

There is therefore a need for improved MOSFET device structures withenhanced stress profile in the channel regions, without increasing theshort channel effects and the junction leakage of the MOSFETs.

SUMMARY OF THE INVENTION

The inventors of the present invention have discovered that source anddrain recesses with slanted sidewall surfaces can be used in conjunctionwith an overlaying stress-inducing dielectric layer to create morestress in the channel regions of MOSFET device structures. The slantedsidewall surfaces of the source and drain recesses function to minimizethe undercut in the source and drain extension regions of the MOSFETdevice structures, which in turn minimizes the short channel effects andthe junction leakage in the MOSFETs.

In one aspect, the present invention relates to a semiconductor devicecomprising a metal-oxide-semiconductor field effect transistor (MOSFET)having source and drain regions that are located in a semiconductorsubstrate. The source and drain regions comprise recesses with one ormore sidewall surfaces that are slanted in relation to an upper surfaceof the semiconductor substrate. A stress-inducing dielectric layer islocated over the slanted sidewall surfaces of the recesses at the sourceand drain regions.

Preferably, but not necessarily, the stress-inducing dielectric layercomprises tensilely or compressively stressed silicon nitride.

The upper surface of the semiconductor substrate is preferably orientedalong one of a first set of equivalent crystal planes, and the one ormore sidewall surfaces of the recesses are preferably oriented along asecond, different set of equivalent crystal planes.

In single crystal semiconductor materials, all lattice directions andlattice planes in a unit cell of a single crystal material can bedescribed by a mathematical description known as a Miller Index. On onehand, the notation [hkl] in the Miller Index defines a crystal directionor orientation, such as the [001], [100], [010], [110], and [111]directions in a cubic unit cell of single crystal silicon. On the otherhand, the crystal planes or facets of a single crystal silicon unit cellare defined by the notation (hkl) in Miller Index, which refers to aparticular crystal plane or facet that is perpendicular to the [hkl]direction. For example, the crystal planes (100), (110), and (111) ofthe single crystal silicon unit cells are respectively perpendicular tothe [100], [110], and [111] directions. Moreover, because the unit cellsare periodic in a semiconductor crystal, there exist families or sets ofequivalent crystal directions and planes. The notation <hkl> in theMiller Index therefore defines a family or set of equivalent crystaldirections or orientations. For example, the <100> directions includethe equivalent crystal directions of [100], [010], and [001]; the <110>directions include the equivalent crystal directions of [110], [011],[101], [−1−10], [0−1−1], [−10−1], [−110], [0−11], [−101], [1−10],[01−1], and [10−1]; and the <111> directions include the equivalentcrystal directions of [111], [−111], [1−11], and [11−]. Similarly, thenotation {hkl} defines a family or set of equivalent crystal planes orfacets that are respectively perpendicular to the <hkl> directions. Forexample, the {100} planes include the set of equivalent crystal planesthat are respectively perpendicular to the <100> directions.

Correspondingly, the term “equivalent crystal planes” as used in thepresent invention refers to a family of equivalent crystal planes orfacets as defined by the Miller Indexes, as described hereinabove.

In a specific embodiment of the present invention, the semiconductorsubstrate comprises single crystal silicon, and the first and secondsets of equivalent crystal planes are selected from the group consistingof the {100}, {110}, and {111} planes of silicon.

In a specific embodiment of the present invention, the MOSFET is ap-channel MOSFET. Correspondingly, the upper surface of thesemiconductor substrate is oriented along one of the {110} planes ofsilicon, and the one or more sidewall surfaces of the recesses areoriented along the {111} planes of silicon.

In an alternative embodiment of the present invention, the MOSFET is ann-channel MOSFET. The upper surface of the semiconductor substrate isoriented along one of the {100} planes of silicon, and the one or moresidewall surfaces of the recesses are oriented along the {111} planes ofsilicon.

The source and drain recesses as mentioned hereinabove can have either atrapezoidal cross-section, i.e., with a bottom surface that is parallelto the upper surface of the semiconductor substrate, or a triangularcross-section, i.e., without any bottom surface.

In a particularly preferred, but not necessary, embodiment of thepresent invention, the source and drain regions of the MOSFET furthercomprise metal silicide layers located over the slanted sidewallsurfaces of the recesses but under the stress-inducing dielectric layer.

The semiconductor substrate as mentioned hereinabove may have asemiconductor-on-insulator (SOI) configuration, i.e., it may comprise(from bottom to top) a base semiconductor substrate layer, a buriedinsulator layer, and a semiconductor device layer. The recesses arelocated in the semiconductor device layer. Alternatively, thesemiconductor substrate may comprise a bulk semiconductor structure withthe recesses located therein.

In another aspect, the present invention relates to a method for forminga semiconductor device. Such a method comprises:

-   -   a. crystallographically etching a semiconductor substrate at        selected source and drain regions of a MOSFET to form recesses        therein, wherein the recesses comprise one or more sidewall        surfaces that are slanted in relation to an upper surface of the        semiconductor substrate; and    -   b. forming a stress-inducing dielectric layer over the slanted        sidewall surfaces of the recesses at the source and drain        regions of the MOSFET.

Preferably, the method further comprises forming metal silicide layersover the slanted sidewall surfaces of the recesses at the source anddrain regions of the MOSFET before formation of the stress-inducingdielectric layer.

Preferably, the crystallographic etching is carried out by a wet etchingstep that employs an etchant selected from the group consisting ofammonia, tetramethylammonium hydroxide, potassium hydroxide,ethylenediamine pyrocatechol (EDP), and a combination thereof. Theseetchants are effective and highly selective for silicon etching, andthey can etch silicon along all crystallographic directions, but atdifferent etching rates along different directions. The differentetching rates along different crystallographic directions are caused bythe crystalline structure of silicon, i.e., some crystal orientationsare more resistant to etching than others. The typical pyramid shapes orV-grooves are produced in a <100>-oriented silicon wafer by one of theabove-mentioned etchants, when the etching reaction proceeds in the<100> direction and stops when the etching front hits the {111}-planes.

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an improved MOSFET device havingtrapezoidal source and drain (S/D) recesses with slanted sidewallsurfaces and a stress-inducing dielectric layer located over the slantedsidewall surfaces of such trapezoidal S/D recesses, according to oneembodiment of the present invention.

FIG. 2 shows a cross-sectional view of an improved MOSFET device havingtriangular source and drain (S/D) recesses with slanted sidewallsurfaces and a stress-inducing dielectric layer located over the slantedsidewall surfaces of such triangular S/D recesses, according to oneembodiment of the present invention.

FIGS. 3-6 are cross-sectional views that illustrate exemplary processingsteps for forming the improved MOSFET device of FIG. 1, according to oneembodiment of the present invention.

FIGS. 7-8 are cross-sectional views that illustrate exemplary processingsteps for forming the improved MOSFET device of FIG. 2, according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-knownstructures or processing steps have not been described in detail inorder to avoid obscuring the invention.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

The present invention provides source and drain (S/D) recesses withslanted sidewall surfaces, which can be used in conjunction with astress-inducing dielectric layer to improve the stress profile in thechannel region of a MOSFET device, but without increasing the shortchannel effects and the junction leakage of the MOSFET device.Specifically, the S/D recesses have sidewall surfaces that are tilted orslanted with respect to an upper surface of the semiconductor substratein which the S/D recesses are located.

FIG. 1 shows a cross-sectional view of an improved MOSFET 2 that hassource region 2S, drain region 2D, and channel region 2C located in asemiconductor substrate 10. The semiconductor substrate 10 has asemiconductor-on-insulator (SOD configuration and comprises, from bottomto top, a base semiconductor substrate layer 12, a buried insulatorlayer 14, and a semiconductor device layer 16. Isolation regions 11 areprovided in the semiconductor substrate 10 to isolate the MOSFET 2 fromadjacent devices.

The semiconductor device layer 16 contains trapezoidal surface recessesthat are located at the source and drain (S/D) regions 2S and 2D. Eachof such trapezoidal S/D recesses has sidewall surfaces 16B that areslanted or tilted with respect to an upper surface 16A of thesemiconductor device layer 16 and a substantially flat bottom surface16C that is parallel to the upper surface 16A of the semiconductordevice layer 16. Optional metal silicide layers 18 and 20 can be formedover the sidewall surfaces 16B and bottom surfaces 16C of thetrapezoidal S/D recesses.

The channel region 2C is located in the semiconductor device layer 16between the S/D regions 2S and 2D. No recess is formed at the channelregion 2C. Instead, a gate stack comprising a gate dielectric layer 22,a gate conductor 24, an optional gate silicide layer 26, and optionalspacers 28 is formed over the upper surface 16A of the semiconductordevice layer 16 at the channel region 2C.

A stress-inducing dielectric layer 30 is formed over the entirestructure, including the slanted sidewall surfaces 16B of thetrapezoidal S/D recesses. Such stress-inducing dielectric layer 30 maycomprise either tensile or compressive stress for correspondinglyapplying tensile or compressive stress to the channel region 2C of theMOSFET 2.

On one hand, the trapezoidal S/D recesses with the slanted sidewallsurfaces 16B improves the stress profile created in the channel region2C of the MOSFET 2 by the stress-inducing dielectric layer 30. On theother hand, the trapezoidal S/D recesses do not undercut thesource/drain extension regions of the MOSFET 2 (which are typicallylocated under the spacers 28 and extend into the channel region 2C) andtherefore do not increase the short channel effects or the leakagecurrent in the MOSFET 2.

Similarly, FIG. 2 shows a cross-sectional view of another MOSFET device4 that is similar to that shown in FIG. 1, except that it containstriangular (instead of trapezoidal) S/D recesses in the semiconductordevice layer 17. Each of the triangular S/D recesses have sidewallsurfaces 17B that are slanted away from the upper surface 17A of thedevice layer 17, but without any flat bottom surface. Thestress-inducing dielectric layer 30 is located over the slanted sidewallsurfaces 17B of such triangular S/D recesses for applying stress (eithertensile or compressive) to the channel region 4C of the MOSFET 4.

The triangular S/D recesses as shown in FIG. 2 also improve the stressprofile created in the channel region 4C of the MOSFET 4 by thestress-inducing dielectric layer 30, but without increasing the shortchannel effects or the leakage current in the MOSFET 4.

Specifically, the MOSFET 2 has S/D regions 2S, 2D and a channel region2C located in a semiconductor substrate 10 and between isolation regions12. A gate stack that comprises a gate dielectric layer 22, a gateconductor 24, a dielectric cap layer 26, and optional sidewall spacers27 and 28 is formed over the channel region 2C.

In a preferred embodiment of the present invention, the semiconductordevice layers 16 and 17 as shown in FIGS. 1-2 comprise single crystalsilicon, and their upper surfaces 16A and 17B are oriented along the{110} planes of silicon. In this manner, the MOSFETs 2 and 4 arepreferably p-channel MOSFETs, so that the channel regions 2C and 4C ofsuch p-channel MOSFETs 2 and 4 are oriented along the {110} planes ofsilicon, which function to enhance the hole mobility in the channelregions 2C and 4C. Further, it is preferred that the stress-inducingdielectric layer 30 overlaying the S/D recesses contain intrinsiccompressive stress, which is in turn applied to the channel regions 2Cand 4C for further enhancing the hole mobility.

In an alternative embodiment of the present invention, the semiconductordevice layers 16 and 17 as shown in FIGS. 1-2 comprise single crystalsilicon, and their upper surfaces 16A and 17B are oriented along the{100} planes of silicon. In this manner, the MOSFETs 2 and 4 arepreferably n-channel MOSFETs, so that the channel regions 2C and 4C ofsuch n-channel MOSFETs 2 and 4 are oriented along the {100} planes ofsilicon, which function to enhance the electron mobility in the channelregions 2C and 4C. The stress-inducing dielectric layer 30 overlayingthe S/D recesses preferably contain intrinsic tensile stress, which isin turn applied to the channel regions 2C and 4C for further enhancingthe electron mobility.

The S/D recesses as shown in FIGS. 1-2 can be readily formed bycrystallographic etching, which etches the semiconductor substrate 10along all crystallographic directions, but at significantly differentrates along different crystal planes or orientations. Therefore, theetch pattern formed by such a crystallographic etching process proceedsalong the fast-etched crystal planes and is eventually terminated by theslowly-etched crystal planes. For example, when the semiconductor devicelayers 16 and 17 of FIGS. 1-2 have upper surfaces 16A and 17A orientedalong the {110} planes of silicon, the crystallographic etching can becarried out using an ammonia- or tetramethyl ammonium hydroxide-basedetching solution, which etches the {110} planes at a much faster ratethan along the {111} planes. Therefore, the etch pattern so formed willterminate by the slowly-etched {111} crystal planes, which are slantedaway from the {110} planes and thereby form the slanted sidewallsurfaces 16B and 17B of the S/D recesses.

FIGS. 3-6 illustrate exemplary processing steps that can be used forfabricating the MOSFET device of FIG. 1, according to one embodiment ofthe present invention.

First, a gate dielectric layer 22 is formed over a semiconductorsubstrate 10, as shown in FIG. 3. A pattern gate stack, which comprisesa gate conductor 24 and a dielectric cap layer 25, is formed over thegate dielectric layer 22.

The semiconductor substrate 10 may comprise a bulk semiconductorstructure, or it may has a semiconductor-on-insulator (SOI)configuration with a base semiconductor substrate layer 12, a buriedinsulator layer 14, and a semiconductor device layer 16, as shown inFIG. 3.

The base semiconductor substrate layer 12 may comprise any suitablesingle crystal semiconductor material, which includes, but is notlimited to: Si, SiC, SiGe, SiGeC, Ge alloys, GaAs, InAs, InP, as well asother III-V or II-VI compound semiconductors. The base semiconductorsubstrate layer 12 may also comprise a layered semiconductor such asSi/SiGe, a silicon-on-insulator (SOI) or a SiGe-on-insulator (SGOI).Preferably, the base semiconductor substrate layer 12 is composed of aSi-containing semiconductor material, i.e., a semiconductor materialthat includes silicon. More preferably, the base semiconductor substratelayer 12 consists essentially of bulk single crystal silicon.Alternatively, the base semiconductor substrate layer 12 may compriseone or more buried insulator layers (not shown) therein. The basesemiconductor substrate layer 12 may be doped, undoped or contain bothdoped and undoped regions (not shown) therein.

The buried insulator layer 14 may comprise any suitable insulatormaterial(s), and it typically comprises an oxide, a nitride, or anoxynitride in either a crystalline phase or a non-crystalline phase. Thephysical thickness of the buried insulator layer 14 typically rangesfrom about 10 nm to about 400 nm, and more typically from about 20 nm toabout 200 nm.

The semiconductor device layer 16 may comprise any single crystalsemiconductor material, which includes, but is not limited to: Si, SiC,SiGe, SiGeC, Ge alloys, GaAs, InAs, InP, as well as other III-V or II-VIcompound semiconductors. Preferably, the semiconductor device layer 16is composed of a Si-containing semiconductor material, i.e., asemiconductor material that includes silicon. More preferably, thesemiconductor device layer 16 consists essentially of single crystalsilicon and has an upper surface 16A that is oriented along one of afirst set of equivalent crystal planes of silicon. In one specificembodiment of the present invention, the upper surface 16A of thesemiconductor device layer 16 is oriented along one of the {110} planesof silicon, as shown by the arrowheads in FIG. 3, so that thesemiconductor device layer 16 can be used for forming a channel regionfor a p-FET device. In an alternative embodiment of the presentinvention, the upper surface 16A of the semiconductor device layer 16 isoriented along one of the {100} planes of silicon (not shown), so thatthe semiconductor device layer 16 can be used for forming a channelregion for an n-FET device. Note that the semiconductor device layer 16and the base semiconductor substrate layer 12 may be formed of the samesemiconductor material or different types of semiconductor materials.

The SOI substrate structure 10 as shown in FIG. 3 can be formed in situby depositing the buried insulator layer 14 over the base semiconductorsubstrate layer 12 via chemical vapor deposition, thermal oxidation or acombination thereof, followed by deposition of the semiconductor devicelayer 16. Alternatively, the SOI substrate structure 10 of FIG. 3 can beformed in situ by a silicon implanted oxide (SIMOX) process, duringwhich oxygen ions are implanted into a bulk semiconductor substrate at apredetermined depth, followed by high temperature anneal to effectuatereaction between the semiconductor material and the implanted oxygenions, thereby forming an oxide layer in the semiconductor substrate atthe predetermined depth. Further, the SOI substrate structure 10 of FIG.3 may be fabricated from pre-formed insulator and semiconductor layersby wafer-bonding or layer transfer techniques.

At least one isolation region, such as, for example, the trenchisolation region 11, can be provided in the semiconductor device layer16 of the SOI substrate 10 to isolate the device region for the MOSFET 2from the adjacent device regions. The isolation region may be a trenchisolation region 11 (as shown in FIG. 3) or a field oxide isolationregion. The trench isolation region 11 is formed utilizing aconventional trench isolation process well known to those skilled in theart. For example, lithography, etching and filling of the trench with atrench dielectric may be used in forming the trench isolation region 11.Optionally, a liner may be formed in the trench prior to trench fill, adensification step may be performed after the trench fill and aplanarization process may follow the trench fill as well. The fieldoxide may be formed utilizing a so-called local oxidation of siliconprocess.

The gate dielectric layer 22 of the present invention may be comprisedof any suitable dielectric material, including, but not limited to:oxides, nitrides, oxynitrides and/or silicates (including metalsilicates and nitrided metal silicates). In one embodiment, it ispreferred that the gate dielectric layer 22 is comprised of an oxidesuch as, for example, SiO₂, HfO₂, ZrO₂, Al₂O₃, TiO₂, La₂O₃, SrTiO₃,LaAlO₃, and mixtures thereof. The physical thickness of the gatedielectric layer 22 may vary widely, depending on the specificdeposition technique employed. Typically, the gate dielectric layer 24has a thickness from about 0.5 to about 10 nm, with a thickness fromabout 1 to about 5 nm being more typical. The gate dielectric layer 22can be formed by a thermal growing process such as, for example,oxidation, nitridation or oxynitridation. Alternatively, the gatedielectric layer 22 can be formed by a deposition process such as, forexample, chemical vapor deposition (CVD), plasma-assisted CVD, atomiclayer deposition (ALD), evaporation, reactive sputtering, chemicalsolution deposition and other like deposition processes. The gatedielectric layer 22 may also be formed utilizing any combination of theabove processes.

The patterned gate stack, which comprises the gate conductor 24 and theoptional dielectric cap layer 25, is formed over the gate dielectriclayer 22, by first depositing a blanket gate conductor layer (not shown)and a blanket dielectric capping layer (not shown) over the gatedielectric layer 22, followed by patterning the blanket gate conductorlayer (not shown) and the dielectric capping layer (not shown) into thegate conductor 24 and the optional dielectric cap layer 25 usingconventional lithography and etching. The lithographic step, preferablyinverse gate level (PC) lithography, includes applying a photoresist(not shown) to the upper surface of the blanket dielectric capping layer(not shown), exposing the photoresist (not shown) to a desired patternof radiation and developing the exposed photoresist (not shown)utilizing a conventional resist developer. The pattern in thephotoresist (not shown) is then transferred to the underneath dielectriccapping layer (not shown), the blanket gate conductor layer (not shown),and the blanket gate dielectric layer (not shown) utilizing one or moredry etching steps. Suitable dry etching processes that can be used inthe present invention include, but are not limited to: reactive ionetching (RIE), ion beam etching, plasma etching or laser ablation.Preferably, but not necessarily, the gate conductor layer 24 comprisespolycrystalline silicon (poly-Si), and the dielectric cap layer 25comprises silicon nitride. The etching step preferably is carried out byRIE techniques. The patterned photoresist (not shown) is then removed byresist stripping after etching has been completed.

Conventional dopant implantation can then be conducted to form S/Dextension regions (not shown) and halo regions (not shown) in thesemiconductor device layer 16 using the patterned gate stack as a mask.Alternatively, the S/D extension and halo implantation can be performedafter the crystallographic etching step.

Subsequently, a blanket dielectric layer (not shown) can be depositedover the entire structure and then patterned into dielectric spacers 28along sidewalls of the gate stack, as shown in FIG. 4. Preferably, thedielectric spacers 28 are formed from a blanket silicon nitride layerand are patterned by reactive ion etching RIE). The dielectric spacers28 may merge with the dielectric cap layer 25 located atop the gateconductor 24 to form a continuous dielectric structure that covers theentire gate conductor 24.

After formation of the dielectric spacer 28, a crystallographic etchingprocess is carried to form surface recesses 13 in the semiconductordevice layer 16 at regions adjacent to the gate stack, as shown in FIG.5.

The crystallographic etching process can be carried out by any suitabledry and/or wet etching techniques known in the art. Preferably, but notnecessarily, the crystallographic etching of the semiconductor devicelayer 16 is carried out by one or more wet-etching processes, whichemploy etching solutions such as ammonia-based etching solutions,tetramethyl ammonium hydroxide (TMAH)-based etching solutions,hydroxide-based etching solutions, ethylene diamine pyrocatechol(EDP)-based etching solutions, etc.

The wet-etching processes typically etch the semiconductor device layer16 along all crystallographic directions, but at significantly differentrates along different crystal planes or orientations. Therefore, theetch patterned formed by the crystallographic etching proceeds along thefast-etched crystal planes and is eventually terminated by theslowly-etched crystal planes. For example, an etching solution thatcomprises approximately 23.4% KOH, 13.3% isopropyl alcohol (IPA), and63.3% water, when heated to about 80° C., etches the single crystalsilicon at an etching rate of about 1.0 μm/minute along the {100}planes, but at an etching rate of about 0.06 μm/minute along the {110}planes. In other words, this etching solution etches the {100} planesabout 17 times faster than the {110} planes. Therefore, such an etchingsolution can be used to etch a silicon substrate with a {100} surface toform a recess that is terminated at the {110} planes. In contrast, anetching solution that comprises approximately 44% KOH and 56% water,when heated to about 120° C., etches the single crystal silicon at anetching rate of about 11.7 μm/minute along the {110} planes, about 5.8μm/minute along the {100} planes, and about 0.02 μm/minute along the{111} planes. In other words, this etching solution etches the {110} and{100} planes significantly faster than the {111} planes (more than 550and 250 times faster, respectively). Therefore, such an etching solutioncan be used to etch a silicon substrate with a {100} or a {110} surfaceto form a recess that is terminated at the {111} planes.

Most preferably, the crystallographic etching of the semiconductordevice layer 16 is carried out by using an aqueous etching solution thatetches a silicon layer along the {110} planes much faster than the {111}planes and can therefore be used to form surface recesses with slantedsidewalls oriented along the {111} planes on a silicon semiconductordevice layer with a {110} upper surface. For more details regarding theetching solution, see O. Weber et al., A Novel Locally Engineered (111)V-channel pMOSFET Architecture with Improved Drivability Characteristicsfor Low-Standby Power (LSTP) CMOS Applications, 2005 VLSI, p. 156.

If the crystallographic etching process proceeds in a controlled mannerand is terminated within a relatively short time period (T1), thesurface recesses 13 each will have a trapezoidal cross section with arelatively flat bottom surface 16C that is parallel to the upper surface16A of the semiconductor device layer 16 and one or more sidewallsurfaces 16B that are slanted or tilted away from the upper surface 16Aof the semiconductor device layer 16, as shown in FIG. 5. Specifically,when the upper surface 16A of the semiconductor device layer 16 isoriented along one of the {110} planes of silicon, the sidewall surfaces16B of the recesses 13 are oriented along the {111} planes of silicon,which are tilted or slanted in relation to the {110} crystal planes.

After formation of the surface recesses 13 in the semiconductor devicelayer 16, a second dopant implantation step can be carried out to formsource and drain implantations (not shown), followed by a hightemperature anneal to active the implanted dopant species. Next, anitride RIE step can be carried out to remove the silicon nitridematerial from over the gate conductor 24, and a salicidation step can besubsequently carried out to form S/D metal silicide layers 18 and 20 anda gate metal silicide layer 26, as shown in FIG. 6. The S/Dimplantation, the nitride RIE and the salicidation steps are well knownin the art and are therefore not described herein.

A stress-inducing dielectric layer 30 is then deposited over the entirestructure to form a complete MOSFET as shown in FIG. 1. Thestress-inducing dielectric layer 30 preferably comprises tensilely orcompressively stressed silicon nitride, which can be readily formed byany suitable dielectric deposition method. Specifically, a compressivelyor tensilely stressed silicon nitride layer can be formed by, forexample, a low pressure chemical vapor deposition (LPCVD) process or aplasma enhanced chemical vapor deposition (PECVD) process, as disclosedby U.S. Patent Application Publication No. 2003/0040158 or by A. Tarrafet al., “Stress Investigation of PECVD Dielectric Layers for AdvancedOptical MEMS,” J. MICROMECH. MICROENG., Vol. 14, pp. 317-323 (2004), orby any other suitable deposition techniques well known in the art, suchas, for example, high density plasma (HDP) deposition technique.Preferably, the compressively or tensilely stressed silicon nitridelayer has a thickness ranging from about 10 nm to about 500 nm, morepreferably from about 20 nm to about 200 nm, and most preferably fromabout 40 nm to about 100 nm.

If the crystallographic etching process as mentioned hereinabove isallowed to proceed for relatively long period of time (T2, where T2>T1),surface recesses 15 as shown in FIG. 7 can be formed in a semiconductordevice layer 17 that has an upper surface 17A. Specifically, the surfacerecesses 15 each has a triangular (instead of trapezoidal) cross sectionwith sidewall surfaces 17B that are slanted or tilted away from an uppersurface 17A of the semiconductor device layer 17, without any flatbottom surface, as shown in FIG. 7. Specifically, when the upper surface17A of the semiconductor device layer 17 is oriented along one of the{110} planes of silicon, the sidewall surfaces 17B of the recesses 15are oriented along the {111} planes of silicon, which are tilted orslanted in relation to the {110} crystal planes. The same etchingsolution as mentioned hereinabove can be used for forming the triangularrecesses 15, except that the etching is allowed to proceed for arelatively long period of time (T2).

After formation of the surface recesses 15 in the semiconductor devicelayer 17, S/D dopant implantation, nitride RIE, and salicidation, asdescribed hereinabove, can be carried out to form the source and drainimplantations (not shown), the S/D metal silicide layers 18 and 20, andthe gate metal silicide layer 26, as shown in FIG. 8. Subsequently, astress-inducing dielectric layer 30 is deposited over the entirestructure to form a complete MOSFET as shown in FIG. 2.

Note that while FIGS. 1-8 illustratively demonstrate exemplary MOSFETdevice structures and exemplary processing steps for forming such devicestructures, according to specific embodiments of the present invention,it is clear that a person ordinarily skilled in the art can readilymodify such a device structure and processing steps for adaptation tospecific application requirements, consistent with the abovedescriptions. For example, while the semiconductor substrates shown inFIGS. 1-8 represent semiconductor-on-insulator (SOI) substrates, itshould be appreciated that bulk semiconductor substrates can also beused for practice of the present application. Further, while the {110}and {111} crystal planes of single crystal silicon are primarilyillustrated by FIGS. 1-8, other suitable crystal planes, such as the{100}, {111}, {211}, {311}, {511}, and {711} planes of single crystalsilicon, can also be used in any suitable combination in the presentinvention, consistent with the spirit and principles describedhereinabove.

It is noted that the drawings of the present invention are provided forillustrative purposes and are not drawn to scale.

While the invention has been described herein with reference to specificembodiments, features and aspects, it will be recognized that theinvention is not thus limited, but rather extends in utility to othermodifications, variations, applications, and embodiments, andaccordingly all such other modifications, variations, applications, andembodiments are to be regarded as being within the spirit and scope ofthe invention.

1. A method for forming a semiconductor device, comprising:crystallographically etching a semiconductor substrate at selectedsource and drain regions of a MOSFET to form recesses therein, whereinthe recesses comprise one or more sidewall surfaces that are slanted inrelation to an upper surface of the semiconductor substrate; and forminga stress-inducing dielectric layer over the slanted sidewall surfaces ofthe recesses at the source and drain regions of the MOSFET.
 2. Themethod of claim 1, wherein the upper surface of the semiconductorsubstrate is oriented along one of a first set of equivalent crystalplanes, wherein the sidewall surfaces of the recesses are oriented alonga second, different set of equivalent crystal planes.
 3. The method ofclaim 2, wherein the semiconductor substrate comprises single crystalsilicon, and wherein the first and second sets of equivalent crystalplanes are selected from the group consisting of {100}, {110}, and {111}planes of silicon.
 4. The method of claim 1, wherein each of therecesses has a trapezoidal cross-section with a bottom surface that isparallel to the upper surface of the semiconductor substrate.
 5. Themethod of claim 1, wherein each of the recesses has a triangularcross-section without any bottom surface.
 6. The method of claim 1,further comprising forming metal silicide layers over the slantedsidewall surfaces of the recesses at the source and drain regions of theMOSFET before formation of the stress-inducing dielectric layer.
 7. Themethod of claim 1, wherein the semiconductor substrate has asemiconductor-on-insulator (SOI) configuration and comprises a basesemiconductor substrate layer, a buried insulator layer, and asemiconductor device layer, and wherein the recesses are located in thesemiconductor device layer.
 8. The method of claim 1, wherein thesemiconductor substrate comprises a bulk semiconductor structure.
 9. Themethod of claim 1, wherein the crystallographic etching is carried outby a wet etching step using an etchant selected from the groupconsisting of ammonia, tetra-methyl ammonium hydroxide, and acombination there